In recent years, for example, like a mobile communication unit such as a cellular phone, or a wireless LAN (Local Area Network) based on the so-called IEEE (Institute of Electronic and Electronics Engineers) 802.11 standard, various wireless communication techniques have been remarkably developed, and in accordance with this, various techniques concerning an antenna element as an inevitably provided member in order to perform wireless communication have also been developed.
As an antenna element, for example, one in which a radiation electrode, a surface electrode or the like is formed on a cylindrical dielectric is known. This kind of antenna element is generally installed at the outside of an equipment body and is used. However, in the antenna element of such a type that it is disposed at the outside and is used, there are problems that miniaturization of the equipment is obstructed, high mechanical strength is required, and the number of parts is increased.
Then, as an antenna element substituting for this, a chip-like antenna element which can be surface-mounted on a printed-circuit board provided in the inside of an equipment body has been proposed.
As the chip-like antenna element, various ones, for example, a so-called reverse F-type antenna in which a conductor as a radiation electrode is formed into a reverse F shape, and a so-called helical antenna in which a conductor is formed into a coil shape, have been proposed. In such a chip-like antenna element, what is formed by using a high dielectric constant material, such as ceramic, as a base member is typical. However, in this kind of antenna element, there are defects that the high dielectric constant material itself is expensive, and the working thereof is troublesome, and there are problems that the productivity lowers and the manufacture cost increases.
Then, in recent years, with the improvement of a photoetching technique, for the purpose of resolving the disadvantages as stated above, a so-called printed antenna is proposed in which a printed-circuit board having copper foils on both sides is used as a base member, and the photoetching technique is used to form an antenna conductor on this (for example, see patent document 1: JP-A-5-347509, and patent document 2: JP-A-2002-118411).
The patent document 1 discloses a printed antenna in which an antenna conductor layer including at least a loop-shaped conductor part is formed by using an upper side copper foil of a both-sided substrate, an earth conductor layer is formed by using a lower side copper foil, and an insulating material part between the upper and lower copper foils of the both-sided substrate is used as a dielectric layer. In this printed antenna, a feeding part is formed of the copper foil at the side of the earth conductor layer while being insulated from the earth conductor layer, and the loop-shaped conductor part of the antenna conductor layer and the earth conductor layer are connected to each other by a grounding conductor through the dielectric layer. Besides, in this printed antenna, a feeding conductor is made to face on the inside of the loop-shaped conductor part from the feeding part through the dielectric layer, and a series resonant circuit including an inductance element and a capacitor element to cancel the reactance of an antenna body part and to broaden the band width is provided between the feeding conductor and the loop-shaped conductor part. The patent document 1 describes that by constructing the printed antenna as stated above, the band width can be broadened by using the reactance compensation method, the total combination adjustment after manufacture can be made unnecessary, and a drop in antenna gain can be reduced.
Besides, the patent document 2 discloses a helical antenna in which plural through holes are formed alternately in parallel on a printed-circuit board, and ends of these through holes are connected so as to form a spiral as a whole. This patent document 2 describes that an antenna element for a small mobile communication unit can be provided by constructing the helical antenna as stated above.
By the way, in recent years, also in a portable electronic equipment such as a so-called personal digital assistance (hereinafter referred to as a PDA), in order to enable access to a network such as the Internet from, for example, a place where one has gone, a wireless communication function such as the foregoing wireless LAN of the IEEE802.11 standard is added.
In such an electronic equipment, since transmission/reception of signals is performed while being carried, there is a fear that polarization planes become different between a transmission side of the signals and a reception side, and there is a case where reception at the reception side becomes difficult. Thus, in the electronic equipments, in order to enable the reception even when the polarization planes are not identical to each other between the transmission side and the reception side, there are many cases where antenna elements for performing transmission/reception of signals of circular polarization, not so-called linear polarization, are mounted.
As the antenna element to enable the transmission/reception of the circularly polarized signals, there is a so-called patch antenna. As a specific example of an electronic equipment on which the patch antenna is mounted, a description will be given while using a PDA 200 whose plan view and sectional view seen from below are shown in FIG. 1.
As shown in the drawing, the PDA 200 has a substantially rectangular chassis and is constructed such that two patch antennas 201a and 201b capable of transmitting/receiving circularly polarized signals are disposed in areas in the vicinities of two corners facing each other on a specified circuit board 202 provided in the inside of the chassis. Incidentally, the two patch antennas 201a and 201b are disposed from the viewpoint of directional diversity, and the PDA 200 may be provided with only one of the patch antennas.
The PDA 200 as stated above can transmit/receive the circularly polarized signals by using the patch antennas 201a and 201b, and is enabled to perform wireless communication without changing the polarization plane between the transmission side and the reception side.
However, in recent years, when an electronic equipment performing wireless communication, including a mobile communication unit such as a PDA, is developed, importance has been attached to miniaturization.
Here, when the PDA 200 is used as an example, the patch antenna 201a, 201b has a main plane of about 20 mm×20 mm and a length of about 4 mm to 5 mm in its thickness direction. Thus, in the PDA 200, in order to mount the patch antennas 201a and 201b having large areas, the degree of freedom in the layout on the circuit board, on which other not-shown various modules must be mounted, is extremely limited, and since the chassis for housing the member having the thickness of about 4 mm to 5 mm must be used, especially the length in the thickness direction becomes large, which has obstructed miniaturization.